Though it all started as a joke, there is a topic in Godiscussions.com about the “ultimate tsumego”. You might think that an empty board with the clue “black to win” is a joke. I do too… if it wasn’t, Go would have been solved.
Amazingly enough, there are a couple of tsumegos with an astonishing level of complexity. Personally, I think there is nothing else I can say, but to show you the image and offer you some resources to enjoy this awesome piece of art.
From the “Igo Hatsuyoron” and believed to be written by Dosetsu Inseki (1646-1719) and kept as a secret to all but a few of his students. Though this specific problem was believed to be lost for centuries due to a fire which took place during the Edo period, a copy of the first edition of the book (which included them all) was recovered and republished in 1982 with the commentaries of Fujisawa Shuko.
You can find more information about his special tsumego HERE, though it’s in French, you’ll find a copy of the SGF file and the solution. You’ll find more information, as well, as the whole collection of tsumegos in the book “Igo Hatsuyoron” at the appropriate section in Sensei’s Library.